This invention relates to an improved solvent developable photopolymerizable composition and its use in a preferred process as a photoresist in making printed circuit boards.
Photosensitive compositions particularly useful as photoresists in making printed circuit boards are well known in the prior art with these compositions conventionally stored in roll form. Illustratively a three ply material is disclosed in U.S. Pat. No. 3,469,982 with the photosensitive composition sandwiched between a support film and a cover sheet. The photosensitive composition contains components of monomer, initiator and a polymeric binder which allows such composition to be negative working (i.e., exposure to actinic radiation hardens the film to make the exposed portion insoluble in a developer). The three ply material is unwound from a roll with the cover sheet removed from contact with the photosensitive composition prior to use in lamination to a substrate, e.g., a copper surface. The laminated composition is imagewise exposed to actinic radiation with a support film stripped from the photosensitive composition before or after the exposure step. The unexposed areas of the starting light-sensitive layer are washed away in a development step to form a resist image of polymeric material. For a solvent developable film a conventional solvent employed is methyl chloroform also known as 1,1,1-trichloroethane. Such a solvent will remove portions of the photosensitive layer which have not been exposed to actinic radiation but will not adversely affect exposed portions. The areas of the substrate surface which are not protected by the remaining areas of laminated material are permanently modified by etching or by plating. Thereafter the photoresist is removed in a stripping process. A conventional stripping solvent for a solvent developable composition is a solution of methylene chloride and methanol.
The present invention allows improved results in development and/or strippability of a negative acting solvent developable film through formulation of the photosensitive composition.